1. it 代替连词 that 引导的从句作形式主语。

1) it + be + 过去分词 + that 从句

It's said that Tom has come back from abroad . 据说汤姆已经从国外回来了。

It was reported that dozens of  children died in the accident . 据报导，数十名儿童在事故中死亡。

可用于该句型的过去分词还有：known , thought , told , believed , hoped , announced ,expected , decided , suggested等，该句型常可以转换成含有宾语从句的复合句或者含有不定式作主补的简单句。如：

 It's said that Tom has come back from abroad . →People say that Tom has come back from abroad . →He is said to have come back from abroad .

2) It + be + 名词词组 + that 从句

It's a pity that he can't swim for his age . 真遗憾，就他的年龄来讲，他竟不会游泳。

It's a wonder that you could finish such a hard task in such a short time . 真奇怪，你竟能在如此短的时间内完成艰巨的任务。

可用于该句型的名词词组还有：an honour , a good thing , a fact , a surprise , good news等。

3) It + be /seem + 形容词 + that 从句

It is likely that they will beat us tonight .今晚很可能他们会赢我们。

It doesn't seem that she will give us a hand .她帮我们的可能性似乎不太大。

可用于此句型的开容词还有；wonderful , true , good , right , wrong , (un)important ,useless , surprising , clear , possible , unusual , lucky , certain 等。

4) It + seem / appear / happen + that 从句

It seems that he enjoys pop songs very much .看来他非常喜欢流行歌曲。

It appears that Tom might change his mind .看来汤姆会改变主意。

It happened that I went out that day .碰巧我那天外出了。

注意，句型4可转换成含不定式的简单句。如上述句子可转换为：

He seems to enjoy pop music very much . / Tom appears to change his mind ./ I happened to go out that day.